A guitar is a mechanism with replaceable gears designed for stepwise changes in the gear ratio of the calculated kinematic chain. They are used mainly in rarely reconfigured circuits with a large range and number of gear ratios of the tuning element of the design circuit. These mechanisms are characterized by their simplicity of design. The main disadvantage of guitars is the complexity of tuning.
The machines use guitars with one, two and three pairs of replaceable gears. A guitar with one pair of replaceable gears (see Fig. 1.2) is used mainly in circuits that do not require precise tuning (tuning elements i v And i s). Guitars with two and three pairs of replaceable gears are used, as a rule, for precise tuning of kinematic chains (tuning elements i x, i y and so on.). In Fig. Figure 2.19 shows guitars with two and three pairs of interchangeable gears.
Guitar with two pairs of wheels (Fig. 2.19, A) consists of a plate 1, an axis 2, a fixing bolt 3 and replaceable gears a, b, c, d. Since the sum of the teeth of the engaged wheels is different for different settings, a groove is provided in the guitar plate that allows you to move the axis 2 and thus engage the replacement wheels c And d various diameters. Bolt 3 fixes the guitar plate in the required position for wheel traction A And V.
To select gears, use a single equation with four unknowns
Where i– gear ratio obtained from the FN; a, b, c, d- the number of teeth of the guitar wheels.
The number of solutions to equation (*) is limited by the following factors:
Available set of replaceable gears;
Adhesion conditions
a + b > c + (15…20) (**); from +d > to + (15…20) (***).
To select replacement gears, the following two methods are mainly used: main and additional.
Rice. 2.19. Guitar Replacement Gears: A– with two pairs
replaceable wheels; b– scan of a guitar with two pairs of interchangeable
wheels; V- guitar with three pairs of interchangeable wheels
Basic method– decomposition into prime factors. Used when i is expressed as a simple fraction, the numerator and denominator of which are decomposed into simple factors convenient for selecting wheels. For example,
Let us assume that the set of replaceable gear wheels of the machine contains wheels with numbers of teeth that are multiples of five from 20 to 100. Then,
We check the adhesion conditions (**) according to the permissible value
30 + 70 = 85 + 15.
It is possible that the gear wheel will cut the driven shaft (Fig. 2.19, b) and, therefore, mounting the wheels is impossible. Let's swap the wheels in the numerator or denominator. For example,
We check the adhesion conditions using the larger permissible value: (**) 85 + 70 > 30 + 20; (***) 30 + 65 > 70 + 20.
The adhesion conditions confirm the possibility of installing the selected replacement gears in the guitar.
Additional method- approximate selection. In this case, the method of continued fractions or, more often, the tabular method is used.
Let according to the setting formula i= 0, 309329. Using the tables (see, for example, M.V. Sandakov and others. Tables for selecting gears: Handbook. - 6th ed. M.: 1988. - 571 p.) we select the simple one corresponding to this decimal fraction fraction. After transformations we obtain the number of teeth of replacement wheels
.
Such gears are found in the normal set of replacement gears, such as gear hobbing machines. We check the adhesion conditions: (**) 21 + 65 > 45 + 20; (***) 45 + 47 > 65 + 20.
In a number of machines, for example gear hobbing machines, as a rule, a wider range of kinematic settings is provided. Therefore, such machines use guitars with three pairs of replaceable gears. In these guitars (Fig. 2. 19, V) an additional pair of gears is used, and its plate has two or three grooves for intermediate axes. To select gears, an equation with six unknowns is used
Gear wheels e And f change much less frequently than wheels a,b,c,d. As a rule, their gear ratio is constant and equal to 1; 1/2; 2. This allows for a given pair of wheels to use only four replaceable gears, for example with numbers of teeth 40, 60, 60, 80.
Wheels a, b, c, d are selected according to the rules for selecting wheels for a two-pair guitar, and one more thing is added to the adhesion conditions
e + f > d + (15…20)
For different groups of machines, the sets of replacement gears are different. However, all sets are created based on the general number of teeth of replacement wheels: 20 – 23 – 25 – 30 – 33 – 34 – 37 – 40 – 41 – 43 – 45 – 47 – 50 – 53 – 55 – 58 – 59 – 60 – 62 – 65 – 67 – 70 – 71 – 73 -75 – 79 – 80 - 83 – 85 – 89 – 90 – 92 – 95 – 97 – 98 – 100 – 105 – 113 – 115 – 120 – 127 - 44 wheels in total.
For screw-cutting lathes, a set of wheels is adopted, the number of teeth of which is a multiple of five (there are 22 wheels in a set).
The set of gears for gear cutting machines is limited to a wheel with 100 teeth. In backing machines, the set of wheels is similar to the general one, but it does not have a wheel with 113 teeth. For milling machines (for setting dividing heads), the set consists of wheels with the number of teeth: 25 – 25 – 30 – 35 – 40 – 50 – 55 – 60 – 70 – 80 – 90 – 100 (12 wheels in total).
Federal State Autonomous Educational
institution of higher education
"St. Petersburg State Polytechnic University"
Institute of Metallurgy, Mechanical Engineering and Transport
________________________________________________________
Department of "Technological processes and equipment of automated engineering production"
Methods for selecting replacement gears for metal-cutting machines
Guidelines for laboratory work
Direction: 03.15.05 – “DESIGN AND TECHNOLOGICAL SUPPORT OF MECHANICAL ENGINEERING PRODUCTION”
Profile: 03/15/05_05 – “Technology, equipment and automation of engineering production
Saint Petersburg
Methods for selecting replacement gears for metal-cutting machines. Guidelines for laboratory work for students in the direction of 03/15/05. Contains a description of the device and methods for setting up replacement gears for guitars.
Compiled by:
Doctor of Technical Sciences, Professor Kalinin E.P.
Candidate of Technical Sciences, Associate Professor Portnov S.V.
Art. teacher Nikitin A.V.
Reviewers:
Guidelines were approved at a meeting of the department “Cutting, machines and tools” “” ________ 20__ protocol No. ___
Scientific editor - Doctor of Technical Sciences, Professor D.V. Vasilkov
1. Purpose of the work
Study of the design and methods of tuning guitars with interchangeable gears.
2. General information about replacement wheel guitars
Replaceable gears are used to change the gear ratios of various kinematic chains. Devices with replaceable gears are called guitars. Depending on the number of pairs of replaceable wheels installed in the guitar, single-pair, double-pair and triple-pair guitars are distinguished. The wheels are installed on the ends of shafts, the axes of which are fixed in space or can be rearranged. Using guitars with an adjustable shaft or axle makes it possible to select replacement gears regardless of the center distance (within certain limits). At the same time, the number of wheels with different numbers of teeth that can be installed in a guitar increases, and the accuracy of selecting the required gear ratio increases.
3. Single-pair guitars
Rice. 1. Diagram of a single-pair guitar
The numbers of teeth of wheels 1 and 2 of a single-pair guitar are determined from the equations:
(1)
(2)
a - pitch distance between axes, mm; m - module, mm.
When designing single-pair guitars, the total number of teeth z c is usually set from the series 60, 72, 90, 120. Since the number of unknowns z 1 and z 2 is equal to the number of equations, the required number of teeth is uniquely determined from these equations. Wheel tooth numbers can only be integers. However, when solving these equations, depending on the values of i 21 and z c, the values of z 1 and z 2 can be obtained in the form of integers or mixed numbers. The latter are rounded to whole numbers. Therefore, it is difficult to obtain a precisely specified gear ratio when using a single-pair guitar in most cases.
Example 1. Determine the number of teeth of the replacement wheels of a single-pair guitar with z c =72 at i 21 = 1/3.
From the equations:
And 
we get: 
And 
, A 
Examination: 
In this case, the numbers of teeth z 1 and z 2 are obtained in the form of integers, since the value z c = 72 is divided without a remainder by the sum of the numerator and denominator (1+3) of the required gear ratio.
Example 2. Determine the number of teeth of the replacement wheels of a single-pair guitar with z c = 72 and i 21 = 0.329.
From the equations: 
And 
we get: 
And 
, A 
We accept: z 1 = 18 and z 2 = 54
Examination: 
With selected wheels, the specified gear ratio is reproduced approximately.
Single-pair guitars are used when the number of required gear ratios is small and when high demands are not placed on the accuracy of a given gear ratio. They are used in the main movement drives of automatic, semi-automatic and special machines, as well as in the feed drives of some machines, for example, gear hobbing machines.
The device of a two-pair guitar with interchangeable wheels.
Axes 1 and 2 have a constant position. The intermediate wheels b and c are mounted on the swing arm. The radial and arc grooves of which allow the installation of gears with different numbers of teeth i = a/b∙c/d
Sets of replacement wheels are usually provided with a metal-cutting machine. There are heel sets (multiples of 5) and even sets (multiples of 4).
The first selection method is to reduce the gear ratio to a simple fraction. The numerator and denominator are then factored into prime factors and multiplied by constant numbers to ultimately match the number of gears in the set
1 = 9/8=3∙3/4∙2 = 3∙(15)/4∙ (15) ∙3∙ (20) / 2∙ (20)
Adhesion condition.
Second method – the method of continued fractions. The ratio of two integers A and B can be represented as a continued fraction
.
where a 0 , a 1 , a 2 ...a n -1 , a n – quotients from division, obtained as follows: first we divide A by B, we get a 0 , then we divide B by the remainder of the first division, etc., t .e. the previous remainder is divided by the next until the last remainder is equal to 0.
Example: 223/137= A/B.
The third digit of the second line is obtained: the first digit of the first is multiplied by 2 the digit of the second line and the first digit of the second is added. The fourth digit of the second line is obtained by multiplying the second digit of the first line by the third digit of the second line and adding the second digit of the second line.
Replacement wheel guitars are selected with varying degrees of accuracy.
Lathe chucks
Based on the number of jaws, chucks are divided into two-, three-, and four-jaw. Two- and three-jaw chucks are self-centering, four-jaw chucks are usually made with independent movement of the jaws and less often - self-centering.
Accessories and accessories for lathes
A very common way of processing parts on lathes is processing in centers. A clamp is put on the workpiece, installed in the centers of the machine, which rests against the faceplate driver. The faceplate rotates with the machine spindle and rotates the part through a clamp.
The designs of the most common centers are shown in the figure.
2) the middle center is used when trimming ends, when the center should not interfere with the exit of the cutter;
3) a center with a ball is used when it is necessary to grind a cone and shift the axis of the part from the axis of the machine;
4) reverse cone - when processing a small diameter and there is no place to make a center hole. They simply make a conical chamfer.
Clamps
Serve to communicate the rotation of the part installed at the centers.
Clamps come with a straight and curved end, which fits into the groove of the driving chuck. The clamps are made with one screw and two (for high cutting forces) there are self-clamping clamps.
Drive chucks are a disk with 4 slots and a threaded hole for screwing onto the end of the spindle.
Movable and fixed rests
Steady rests are used as additional supports when processing non-rigid shafts. Are used universal fixed or movable rests with sliding cams and special, designed for processing certain parts.
Methods for processing conical surfaces
Some metal-cutting machines use devices called guitars to adjust kinematic chains (see figure 11, d). They provide the necessary grip on the replacement gears. To implement precise gear ratios, two-pair and three-pair guitars are used. Each guitar comes with a specific set of replacement wheels.
Normal sets of replacement gears are shown in Table 4. In order for the selected replacement gears to fit on the guitar and not rest against the bushings of the gear shafts, the following engagement conditions must be observed: a+b^H-(15-22); s+e^e+(15-22).
The sums of the numbers of the mating wheels should not exceed the permissible value determined by the design and dimensions of the space allocated for placing the guitar on the machine.
There are several ways to select the number of teeth of replacement gears.
Method of factorization are used if the numerator and denominator of the gear ratio obtained from the adjustment equation can be decomposed into them.
Having carried out the decomposition, they reduce the fraction or introduce additional factors, combining them so as to obtain an expression of the fraction in terms of the number of teeth available in the set of replacement wheels.
 |
A way to replace common numbers with approximate fractions is that frequently occurring numbers 
replace
approximate values (Table 7), making it possible to obtain gear ratios with sufficient accuracy. This method is used on screw-cutting lathes when it is necessary to cut modular or pit threads, as well as when cutting inch threads if there is no wheel with the number of teeth z = 127 in the set.
EXAMPLE 2. Select replacement gears for cutting inch threads with the number of threads per inch k = 10 on a screw-cutting lathe with a screw pitch px, in = 6 mm and a constant gear ratio 1 P post = 1-
We solve this example using Table 7:
When using approximate methods for selecting replacement wheels, the resulting gear ratio differs from the specified one, so there is a need to determine the adjustment error. For example, in our case
The absolute error will be equal to 0.42333-0.42307=0.00026
For example, for the gear ratio 
In the corresponding column of the tables V.A. Shishkov (see Table 8) we find a close value of the logarithm lg i, which corresponds to the replaceable gear wheels of a guitar with a gear ratio
Table 6 gives values of gear ratios less than one, so for i>l you need to take the logarithm of the reciprocal! transmission ratio values:
 |
Selection of numbers of wheel teeth using a slide rule. The edge of the slide rule slider is set against the number corresponding to the gear ratio. By moving the viewfinder, you find the marks that coincide on the slider and on the ruler. The risks must correspond to integers that, when divided, give the value of the gear ratio. Then the number of teeth of the replacement gears is selected, for example, by factoring into simple factors:
Leaving the slider in the obtained position, we move the viewfinder until the marks on the slider and on the ruler coincide. Then
As a rule, this method of selecting wheels when cutting threads cannot be used, since its accuracy is usually low.
Selection of the number of teeth according to the tables of M.V. Sandakova. Very often, the gear ratio contains fractional numerators and denominators or multipliers that are not multiples of the set of wheels. In this case, it is convenient to select the number of teeth of gear wheels according to the tables of M.V. Sandakov, containing 100,000 gear ratios. The given gear ratio in the form of a simple proper fraction, inconvenient for conversion, must first be converted into a decimal fraction with six decimal places. If the fraction is an improper fraction, you must divide the denominator by the numerator to get a decimal fraction less than one. After this, a decimal fraction equal to or closest to the received one is found in the table, and next to it is a simple fraction corresponding to it. Having received a simple fraction, then
the number of teeth of replacement wheels is selected in the usual way, for example, from -
Guitar- a machine unit designed to change the feed speed. Interchangeable wheel guitars allow you to adjust the pitch with a fair degree of precision.
a, b, c, d- number of teeth of replacement wheels.
To select the correct replacement wheels, you must: adhesion condition.
a + b>c + 22- these 2 equations must be satisfied
с + d > в + 22
Each guitar comes with a specific set of replacement gears.
Replacement wheels are selected in various ways. The easiest way is factorization.
The adhesion condition is fulfilled.
2.7.6. Lathes of the turning group: screw-cutting lathes, rotary lathes, turret lathes, multi-cutting lathes, backing lathes, single-spindle and multi-spindle automatic and semi-automatic machines. Purpose, layout, main components and characteristic parameters, movements, indices.
Designed for processing external, internal, end surfaces of rotating bodies, as well as cutting threads (metric, inch, modular, pitch, special).
Metric thread- with pitch and basic thread parameters in fractions meters.
Inch thread- all thread parameters are expressed in inches (most often indicated by a double stroke placed immediately after the numerical value, for example, 3" = 3 inches), thread pitch in fractions inches(inch = 2.54 cm). Metric and inch threads are used in threaded connections and screw drives.
Modular thread- thread pitch is measured module(m). To get the size in millimeters, just multiply the module by pi (π). Pitch thread- thread pitch is measured in pitches(p"). To obtain the numerical value (in inches), it is enough to divide the number pi (π) by the pitch. Modular and pitch threads are used when cutting the worm of a worm gear.
All kinds of cutting tools are used incisors– for turning, turning, threading. In addition, for thread cutting - taps(internal) and dies(external). For making holes - drills, countersinks, reamers.
Lathes and screw-cutting lathes The most common type of turning group. The characteristic dimensions of lathes are the maximum machining diameter above the bed, which ranges from 100 to 6300 mm, and the maximum machining length (from 125 to 20,000 mm). The kinematic structure of lathes contains kinematic chains for driving spindle rotation and driving longitudinal and transverse feed. The reversal of the spindle is performed by an electric motor, and the activation and reversal of feeds is carried out by mechanisms located in the apron. Rotary slide movement used for turning cones and quills
When using a lathe-screw-cutting machine, a screw-cutting form-building kinematic chain is added that connects the rotation of the spindle with the longitudinal feed from the lead screw. In this case, the feed is turned on by a split nut MG. Reversing the spindle together with the screw-cutting chain in this case, in most machines, is transferred from the electric motor to a special reversing mechanism with friction clutches, because When cutting threads, reversing is required frequently.
Example of a lathe designation:
Automatic lathes, automatic lathes. Lathes operating in automatic and semi-automatic modes are intended for processing various surfaces of rotating bodies from piece or rod workpieces. Highly efficient technological methods for processing elementary surfaces are widely used here: processing with wide cutters with transverse feed, turning external and internal surfaces with shaped cutters, the use of thread-cutting heads, etc. Single-spindle and semi-automatic machines are classified as the first type, and multi-spindle machines are classified as the second type of machines of the first type. (turning) group, which is reflected in the designation of the models of these machines: 1112, 1B125, 1D118, POZA, 1Sh6P, 1A225-6, 1K282, 1A240P-8, 1265M-8, 1283, etc. When manufacturing any identical parts on In this machine, a so-called work cycle must be distinguished, i.e., periodic repetition of individual actions and movements. During the working cycle, an automatic or semi-automatic machine processes one part.
Multi-spindle automatic lathe
Turning and turret. They are designed for turning in mass production of parts with complex configurations using various tools, most of which are fixed in the turret. Turret lathes are divided into bar and cartridge lathes. Almost all basic turning operations can be performed on these machines. The use of these machines is considered rational if the technological process of processing a part requires the sequential use of various cutting tools: cutters, drills, reamers, taps, etc. The tools are secured in the required sequence in the corresponding positions of the turret and the tool holders of the transverse supports. A lead screw is not required for turret machines, because the threads on them are cut only with taps or dies. In turret lathes, rotation speed and feed are switched mainly using command devices. The main dimensions characterizing bar turret machines are the largest diameter of the part processed in the chuck above the bed and above the support. The main dimensions include the maximum distance from the front end of the spindle to the front edge or end of the turret and the maximum movement of the head.